[mlgo][regalloc] Handle training case when no regalloc happens.

There's an early-exit case for regalloc when we don't even get a chance
to ask for an advisor (priority or eviction), and switch the context.
Then, when we want to log the reward for that function (==the one with
the early exit case), we hit the error case where the function's name
doesn't match the last-seen context.

There are a few possible fixes, one would be to just switch context when
output-ing the reward, which would be correct. This patch opts for the
alternative where we check any loging happened in the first place - just
to re-validate that no function would have been regaloc-ed without first
log-ing its reward.

Differential Revision: https://reviews.llvm.org/D143359